Fin-type antifuse

ABSTRACT

A method of forming an antifuse forms a material layer and then patterns the material layer into a fin. The center portion of the fin is converted into a substantially non-conductive region and the end portions of the fin into conductors. The process of converting the center portion of the fin into an insulator allows a process of heating the fin above a predetermined temperature to convert the insulator into a conductor. Thus, the fin-type structure that can be selectively converted from an insulator into a permanent conductor using a heating process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a method of forming an antifuse andthe resulting structure which includes a fin structure that can beconverted from an insulator into a permanent conductor through a heatingprocess.

2. Description of the Related Art

Fuses and antifuses are useful in today's integrated circuit devices toselectively connect and disconnect devices from other portions of thecircuit, as well as to provide logical operations. For example, a fuseis often activated (blown, opened, etc.) in order to disrupt or break anelectrical connection. Similarly, a fuse can be blown to dramaticallyincrease the resistance of a circuit, thereby providing a logicaldistinction between the activated and unactivated fuse device.

Antifuses operate in an opposite manner to that of fuses. Thus,antifuses are generally non-conductive (highly resistive) whenunactivated (unblown) and become conductors when activated (blown).Therefore, when an antifuse is activated, it forms an electricalconnection, as opposed to a fuse which breaks an electrical connectionwhen activated. Thus, an antifuse selectively allows a conductiveconnection to be made to selectively connect portions of a circuittogether, thereby potentially engaging a previously disconnected deviceinto a circuit. Similarly, an antifuse provides different resistancevalues which can be utilized to perform logical operations.

Once a fuse or antifuse is activated, the fuse generally cannot beunactivated. Therefore, the activation is generally a one-time event andis used to permanently modify a circuit. Low process cost and relativelyhigh density are required for fuses and antifuses. Electrically blowingmetal fuses is one conventional method for activating a fuse, butrequires precise electrical and physical control to be reliable. Theinvention described below provides much smaller and easily activatedantifuses and methods for making the same.

SUMMARY OF THE INVENTION

This disclosure presents a method of forming an antifuse and theresulting structure. The invention forms a material layer and thenpatterns the material layer into a fin. Next, the invention converts thecenter portion of the fin into a high-resistance conductor and the endportions of the fin into conductors. The process of converting thecenter portion of the fin into a high-resistance conductor(substantially non-conductive) allows a process of heating the fin abovea predetermined temperature to convert the high-resistance conductorinto a low-resistance conductor (substantially conductive). Thus, theinvention provides a fin-type structure that can be selectivelyconverted from a high-resistance conductor into a low-resistanceconductor using a simple heating process.

In the process of converting the center portion of the fin into ahigh-resistance conductor, the ends of the fin are masked such that thecenter portion of the fin is unprotected, and then ions are implantedinto the center portion of the fin. This process changes the centerportion of the fin into an amorphous material. Thus, for example, thisprocess of converting the center portion of the fin into ahigh-resistance conductor changes the center portion from single-crystalsilicon to amorphous silicon. The subsequent selective heating of thefin changes the center portion of the fin into polycrystalline silicon.The selective heating of the fin that activates the antifuse changes theconductivity level of the center portion of the fin to be many timesmore conductive (e.g., 10 times more conductive) after being heatedabove the predetermined temperature when compared to the conductivitylevel of the center portion before heating. In a similar manner, theprocess of converting the end portions of the fin into conductors cancomprise protecting the center portion of the fin, and then silicidingthe end portions of the fin.

In another embodiment, the invention forms a directional antifuse thathas a preferred direction of condition (bias) before and after beingactivated. This embodiment again patterns a material layer into a fin.The end portions of the fin are converted into a P-type end and anN-type end and the center portion of the fin is converted into a P-Njunction. This process of converting the center portion of the fin intothe P-N junction allows a process of heating the fin above apredetermined temperature to permanently change characteristics of theP-N junction.

More specifically, the process of converting the center portion of thefin into a P-N junction comprises masking ends of the fin such that thecenter portion of the fin is unprotected. Then ions are implanted intothe center portion of the fin. The process of implanting ions into thecenter portion of the fin changes the center portion of the fin into anamorphous material. The process of converting the end portions of thefin into conductors comprises protecting the center portion of the fin,protecting the N-type end and implanting P-type impurities into theP-type end, and protecting the P-type end and implanting N-typeimpurities into the N-type end. These N-type impurities and the P-typeimpurities comprise opposite type impurities. After the P-type andN-type impurities are implanted into the ends of the fin, the fin isheated sufficiently to drive impurities from the ends of the fin intothe center of the fin.

The heating of the fin decreases the density of mid-gap states of thecenter portion of the fin by, typically two to three orders ofmagnitude, compared to the level of the center portion before heating.Thus currents across the P-N junction when reverse biased are decreasedby two or more orders of magnitude (e.g., suppress by 100× or more)after heating.

One of the antifuse structures provided by the invention comprises a finhaving a center portion and end portions. The center portion of the fincomprises an insulator adapted to permanently become a conductor whenheated above a predetermined temperature and the end portions comprisepermanent conductors.

In this structure, the center portion of the fin comprises an amorphousmaterial that is many times more conductive after being heated above thepredetermined temperature when compared to a conductivity level of thecenter portion before heating. For example, the center portion cancomprise single crystal silicon before being heated above thepredetermined temperature and is changed to polycrystalline siliconafter being heated above the predetermined temperature. The end portionscomprise a permanent conductor, such as silicide regions of the fins.

The size (length) of the center portion of the fin is restricted suchthat, for example, the center portion comprises a very small portion(less than approximately 10 percent) of the length of the fin. Further,the fin is a rectangular structure that extends from a substrate and ismore than a conventional rectangular wire. Thus, for example, the finhas a height and length that exceeds more than many times (e.g., 2 ormore times) the width of the fin.

In the embodiment of the antifuse that has a bias (similar to a diode),the center portion of the fin comprises a P-N junction adapted topermanently change characteristics when heated above a predeterminedtemperature and the end portions comprise a P-type end and an N-typeend. Again in this embodiment, the center portion of the fin cancomprises an amorphous material, which can comprise, for example, singlecrystal silicon before being heated above the predetermined temperatureand polycrystalline silicon after being heated above the predeterminedtemperature.

These and other aspects of embodiments of the invention will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingpreferred embodiments of the invention and numerous specific detailsthereof, is given by way of illustration and not of limitation. Manychanges and modifications may be made within the scope of theembodiments of the invention without departing from the spirit thereof,and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be better understood from thefollowing detailed description with reference to the drawings, in which:

FIG. 1 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 2 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 3 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 4 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 5 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 6 is a schematic diagram of a completed antifuse according to theinvention;

FIG. 7 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 8 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 9 is a schematic diagram of a partially completed antifuseaccording to the invention;

FIG. 10 is a schematic diagram of a completed antifuse according to theinvention;

FIG. 11 is a schematic diagram of inventive antifuse used within acircuit; and

FIG. 12 is a flow diagram illustrating the inventive methodology.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention presents a method of forming an antifuse and the resultingstructure. The inventive method and structure produce antifuses that usefin technology. Therefore, the inventive antifuses are smaller thanconventional fuses. Thus, the invention takes advantage of the fact thata fin of silicon is of very low mass (fractions of a picogram) and thusis easily heated electrically. The invention provides an inexpensivetechnique to make the antifuse change resistance by a large amount(e.g., 10×) permanently after a pulse of heating current is passedthrough the fin. Because of the small area (mass) of the inventivestructure, the operation of activating (blowing) the antifuse involves asimplified heating process that is performed by passing current throughthe antifuse. Therefore, not only does the invention produce smallerantifuses than are conventionally available, the invention also providesa method of activating the antifuses that avoids the problems that canoccur during conventional physical/optical antifuse activation.

FIGS. 1-3 illustrate some non-limiting ways in which the fin structuremay be formed. As shown in FIG. 1, the invention forms a material layer102 (such as single crystal silicon, silicon-germanium, etc.) on asubstrate 100 (a silicon wafer, a SOI wafer, etc.). A mask 104 (such asan organic photoresist, etc.) is patterned over the material 102 and acommon material removal process such as etching, chemical treatment,etc. is used to remove the exposed portion of the material 102 to leavea freestanding fin 200. Note that the fin 200 is a rectangular structurethat extends from the substrate 100 and is more than a conventionalrectangular wire. Thus, for example, the fin has a height (h) and length(l) that exceeds more than 2 times the width (w) of the fin.

An alternative method to form the fin 200 is shown in FIG. 3. Thismethod is sometimes referred to as sidewall spacer technology. In thismethod, a placeholder 300 is patterned on the substrate 100 usingconventional techniques such as photolithography. Then, the material 102is deposited over the placeholder 300. Next, a selective directionaletching process is utilized to remove the material 102 from horizontalsurfaces at a higher rate than it removes material from verticalsurfaces. This leaves the material 102 only on the side walls of theplaceholder 300. The upper surface of the structure is then preferablypolished to remove any excess material 102, leaving the fin structure200 shown in FIG. 3. After this, the placeholder 300 is removedresulting in the structure shown in FIG. 2.

FIG. 4 illustrates the process of converting the center portion of thefin 200 into an insulator. In FIG. 4, the previous masks are removed andthe ends of the fin are protected using another mask, such as aphotoresist 400. The center portion of the fin is left unprotected.Then, high-mass ions 402 (such as silicon, germanium, arsenic, xenon,etc.) are implanted into the center portion of the fin 200. This ionimplant 402 is sufficient to make the central portion of the fin 200 andamorphous insulating material. For example, the ion implant energies canbe between approximately 0.5 keV and approximately 3 keV with a normaldosage in the range of 1×10¹⁴ cm⁻² to 5×10¹⁴ cm⁻². Once again, thisprocess changes the center portion of the fin 200 into an amorphousmaterial. Thus, for example, this process of converting the centerportion of the fin into an insulator changes the center portion of asilicon fin into a region of amorphous silicon.

In FIG. 5, the previous masks are removed and another mask 500, that issimilar to the previous masks discussed above, is patterned to protectthe center portion of the fin 200 and allow the ends of the fin to beexposed. The exposed portions of the fin are converted into conductorsby implanting an impurity in sufficient quantity to make the ends of thefin 200 conductive. Alternatively, the ends of the fins 200 can besilicided 502 in a process that heats the fin in a metal containingambient. In addition, as would be understood by one ordinarily skilledin the art given this disclosure, many additional methods can beutilized to make the ends of the fin 200 conductive. Then the mask 500is removed leaving the structure shown in FIG. 6 that includes ahigh-resistance center section 600 and conductive ends 602, 604.

Thus, the invention converts the center portion 600 of the fin into ahigh-resistance (substantially non-conductive) region and the endportions 602, 604 of the fin into conductors. The process of convertingthe center portion of the fin into a high-resistance region allows asubsequent process of heating the fin above a predetermined temperatureto convert the high-resistance region into a low-resistance region.Thus, the invention provides a fin-type structure that can beselectively converted from a high-resistance conductor into a permanentlow-resistance conductor using a simple heating process.

In another embodiment, shown in FIGS. 7-10, the invention forms adirectional antifuse that has a preferred direction of conduction (bias)before and after being activate. This antifuse operates as a P-Njunction in a similar manner that a diode operates, except that theactivation of the P-N junction within the fin 200 changes thecharacteristics of the P-N junction dramatically allowing the antifuseto be easily utilized as a logical device. In this embodiment, the endportions of the fin are converted into a P-type end and an N-type endand the center portion of the fin is converted into a P-N junction. Thisprocess of converting the center portion of the fin into the P-Njunction allows a process of heating the fin above a predeterminedtemperature to permanently change characteristics of the P-N junction.

More specifically, as shown in FIG. 7, the process of converting thecenter portion of the fin 200 into a P-N junction comprises masking endsof the fin 200 (again using a mask such as mask 400) such that thecenter portion of the fin 200 is unprotected. Then ions, preferably ofthe silicon or germanium with a normal dosage in the range of 1×10¹⁴cm⁻² to 5×10^(14 cm) ⁻², are implanted into the center portion of thefin as indicated by item 700. The process of implanting ions into thecenter portion of the fin changes the center portion of the fin into anamorphous or polycrystalline semiconductor with a high density ofmid-gap states.

The density of mid-gap states in the junction of a P-N diode stronglyinfluences two important characteristics of the electrical conduction ofsuch a diode, namely, the reverse-bias leakage, and the forward-biasideality. The reverse-bias leakage is typically a very low currentdensity (˜1 pA/cm²), however, when a high density of mid-gap states isintroduced into the junction region, this reverse-bias current canincrease many factors of ten. The forward-bias conduction current growsexponentially with forward voltage with an exponential factor ofenkT/Qe, where k and Qe are Boltzmann's constant and the unit ofelectric charge, respectively, T is the temperature of the junction an nis a number, typically between 1 and 2, called the ideality factor. Forlow densities of mid-gap states n˜1, while for large densities ofmid-gap states, n˜2.

The process of converting the end portions of the fin into N-type andP-type regions comprises a two-step process that protects one of theends and the center portion of the fin while the other end is implantedwith a doping impurity. More specifically, as shown in FIG. 8, a mask800 is formed over one end of the fin 200 and over the center section ofthe fin 200, leaving only the other end of the fin exposed. Then animpurity 802 is implanted into the exposed end of the fin. Similarly, asshown in FIG. 9, the first mask 800 is removed and a second mask 900used to cover the previously exposed portion of the fin 200 and thecenter portion of the fins 200. Then, an opposite type of dopingimpurity 902 is implanted into the exposed portion of the fin. Ifdesired, additional masking can be performed in this process. Forexample, an additional mask similar to the mask 500 shown in FIG. 5,above, can be used in conjunction with this process. After the mask 900is removed, the fin 200 will include a first type of impurity region 112(e.g., P-type), a center highly resistive amorphous region 110, andopposite type doped impurity region 114 (e.g., N-type).

Thus, this process involves protecting the N-type end 114 and implantingP-type impurities 802 into the P-type end 112, and protecting the P-typeend 112 and implanting N-type impurities 902 into the N-type end 114.These N-type impurities and the P-type impurities comprise opposite typeimpurities. For example, the N-type impurities 902 can comprise arsenic,phosphorus, etc. while the P-type impurities 802 can comprise boron,etc. As would be understood by one ordinarily skilled in the art inlight of this disclosure, the actual impurities that are used can changedepending upon the materials used in the antifuse. Indeed, anycombination of substances can be used so long as those substances causethe fin to act in a biased manner such that current is encouraged toflow primarily in one direction. After the P-type and N-type impuritiesare implanted into the ends of the fin, the fin is heated sufficientlyto activate the impurities, typically using anneals of 800° C. to 1050°C. for between 1 s to 30 s. As mentioned above, in order to activate thefuse, current is passed through the fin, which causes the fin to heat. Aforward voltage of between 1 and 2 Volts is applied to the diode for0.01 s to 5 s to achieve temperatures of between 500° C. and 700° C.,which are typically sufficient to accomplish the structural change tolarger polycrystalline semiconductor grains and a lower average densityof mid-gap states.

Therefore, after the antifuse is manufactured, it is utilized within acircuit to form a connection or perform a logical function. FIG. 11illustrates a simplified schematic diagram of the inventive antifuse 117electrically connected by wiring to two items, item A 115 and item B119. Items A and B can comprise any type of item, such as a voltagesource, controller, transistor, capacitor, etc. In one example, it maybe desirable to connect item A to item B. In this case, the antifuse 117would comprise the embodiment shown in FIG. 6 and sufficient current orvoltage would be generated through the fin 200 to heat the centerportion 600 fin to a sufficient temperature to change the center portion600 to a conductor. This would make the antifuse a permanent conductorbetween items A and B. If item A were not to be connected to item B, theantifuse would not undergo the heating process.

Alternatively, the antifuse 117 could represent the antifuse shown inFIG. 10 and item A could represent a logical device and item B couldcomprise a voltage source. In its unactivated state, the antifuse 117would provide one type of characteristic, namely the reverse-biasconduction current would be 100 to 1000 times that of the heated(annealed) antifuse. Thus, before the center portion of said fin isheated, the center portion of said fin has a reverse-bias leakage thatis more than 100 times higher than the reverse-bias leakage after thecenter portion of said fin is heated. Additionally, the electricalconduction in the regime of 50 mV to 350 mV forward bias, would becharacterized by an ideality factor, n=2, which could represent onelogical value. If this logical value were to be changed to an oppositelogical value, the antifuse could be activated as described above byheating. Then, after activation, the antifuse would provide the oppositetype of characteristic, namely lower reverse-bias conduction (100 to1000 times lower reverse-bias conduction) or a forward-bias idealityfactor, n=1, thereby providing item A with a different logical answerwhen accessing the antifuse 117. In the case that the antifuse in 117represents the antifuse shown in FIG. 6, the high-resistance state wouldinitially exist due to the amorphous region 600 and could represent onlogical value. A voltage would be provided on the antifuse whereprogramming is desired, to heat the amorphous region 600, therebyconverting the region 600 to a low-resistance (conductor) state, whichcould represent a second logical value.

With respect to the heating that occurs within the center portion of600, 110 of the fin 200, a narrower center portion allows a lowervoltage to be utilized when performing the heating operation. Therefore,the masks 400 that are utilized should be patterned to make the centerportion 600, 110 of the fin 200 and small as possible along the lengthof the fin 200. This requirement is balanced against the need for thecenter portion of the fin 200 to remain a high-resistance region beforethe antifuse is activated. Therefore, while each design will havedifferent parameters, the length of the center portion 600, 110 of thefin 200 should be minimized to the greatest extent possible to reducethe amount of voltage required to generate the necessary heating. Thus,in one example, the size (length) of the center portion of the fin isrestricted such that, for example, the center portion comprises lessthan approximately 5 to 10 percent of the length of the fin.

The subsequent selective heating of the fin changes the center portionof the fin into polycrystalline silicon. The selective heating of thefin that activates the antifuse in the embodiment shown in FIG. 6,changes the conductivity level of the center portion of the fin to bemany times (e.g., 10 or more times) more conductive after being heatedabove the predetermined temperature when compared to the conductivitylevel of the center portion before heating. In the embodiment shown inFIG. 10, the amorphization of the fin increases the mid-gap density ofstates of the center portion of the fin to be many times (e.g., 100 ormore times) that found in the unamorphized fin. Heating of thisamorphized region decreases the density of states to a value nearby thatof the original unamorphized fin, in the range of <10¹⁷cm⁻³eV⁻¹.

FIG. 12 illustrates the methodology of the invention in flowchart form.More specifically, in item 120, the invention forms a material layer andthen patterns the material layer into a fin in item 122. Next, theinvention converts the center portion of the fin into an amorphousmaterial (124) and converts the end portions of the fin into conductors(126) for them embodiment shown in FIG. 6, or dopes the end portions ofthe fin using opposite type dopants (128), for the embodiment shown inFIG. 10. For the embodiment shown FIG. 10, the structure is thenannealed to drive the dopants into the central region of the fin in item130. The process of converting the center portion of the fin into anamorphous material allows the subsequent selective heating process 132to convert the amorphous center portion of the fin into a conductor.Thus, the invention provides a fin-type structure that can beselectively converted from a high-resistance region into a permanentlow-resistance region, or conductor, using a simple heating process.

The invention presents a method of forming an antifuse and the resultingstructure. The inventive method and structure produce antifuses that usefin technology. Therefore, the inventive antifuses are smaller thanconventional fuses. Thus, the invention takes advantage of the fact thata fin of silicon is of very low mass (fractions of a picogram) and thusis easily heated electrically. The invention provides an inexpensivetechnique to make the antifuse change resistance by a large amountpermanently after a pulse of heating current is passed through the fin.Because of the small area (mass) of the inventive structure, theoperation of activating (blowing) the antifuse involves a simplifiedheating process that is performed by passing current through theantifuse. Therefore, not only does the invention produce smallerantifuses than are conventionally available, the invention also providesa method of activating the fuses that avoids the problems that can occurduring conventional physical/optical antifuse activation.

Benefits which accrue include high-density and low-power encoding ofdata, reduced cost for repair/replacement of redundant elements, abilityto self-heal or self-program elements on a chip in the field, andincreased versatility for use of circuits. These inventive methods andstructures may apply to logic, memory, including SRAM, DRAM, and NVRAM,as well as analog and other integrated circuits.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without departing from the generic concept.Therefore, such adaptations and modifications should and are intended tobe comprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology employed herein is for the purpose of description and not oflimitation. Therefore, while the invention has been described in termsof preferred embodiments, those skilled in the art will recognize thatthe invention can be practiced with modification within the spirit andscope of the appended claims.

1. An antifuse structure comprising: a fin having a center portion andend portions, wherein said center portion of said fin comprises asubstantially non-conductive region adapted to permanently become aconductor when heated above a predetermined temperature, wherein saidend portions comprise conductors.
 2. The antifuse in claim 1, whereinsaid center portion of said fin comprises an amorphous material.
 3. Theantifuse in claim 1, wherein said center portion of said fin isapproximately 10 times more conductive after being heated above saidpredetermined temperature when compared to a conductivity level of saidcenter portion before heating.
 4. The antifuse in claim 1, wherein saidcenter portion comprises less than approximately 10 percent of thelength of said fin.
 5. The antifuse in claim 1, wherein said centerportion comprises amorphous silicon before being heated above saidpredetermined temperature and comprises polycrystalline silicon afterbeing heated above said predetermined temperature.
 6. The antifuse inclaim 1, wherein said end portions comprise silicide regions of saidfin.
 7. The antifuse in claim 1, wherein said fin has a height andlength that exceeds more than 2 times a width of said fin. 8-26.(canceled)
 27. An antifuse structure comprising: a fin having a centerportion and end portions, wherein said fin has a height and length thatexceeds a width of said fin, wherein said center portion of said fincomprises a substantially non-conductive region adapted to permanentlybecome a conductor when heated above a predetermined temperature, andwherein said end portions comprise conductors.
 28. The antifuse in claim27, wherein said center portion of said fin comprises an amorphousmaterial.
 29. The antifuse in claim 27, wherein said center portion ofsaid fin is approximately 10 times more conductive after being heatedabove said predetermined temperature when compared to a conductivitylevel of said center portion before heating.
 30. The antifuse in claim27, wherein said center portion comprises less than approximately 10percent of the length of said fin.
 31. The antifuse in claim 27, whereinsaid center portion comprises amorphous silicon before being heatedabove said predetermined temperature and comprises polycrystallinesilicon after being heated above said predetermined temperature.
 32. Theantifuse in claim 27, wherein said end portions comprise silicideregions of said fin.
 33. The antifuse in claim 27, wherein said heightand said length of said fin exceed more than 2 times said width of saidfin.
 34. An antifuse structure comprising: a fin having a center portionand end portions, wherein said fin has a height and length tat exceedmore than 2 times a width of said fin, wherein said center portion ofsaid fin comprises a substantially non-conductive region adapted topermanently become a conductor when heated above a predeterminedtemperature, and wherein said end portions comprise conductors.
 35. Theantifuse in claim 34, wherein said center portion of said fin comprisesan amorphous material.
 36. The antifuse in claim 34, wherein said centerportion of said fin is approximately 10 times more conductive afterbeing heated above said predetermined temperature when compared to aconductivity level of said center portion before heating.
 37. Theantifuse in claim 34, wherein said center portion comprises less thanapproximately 10 percent of the length of said fin.
 38. The antifuse inclaim 34, wherein said center portion comprises amorphous silicon beforebeing heated above said predetermined temperature and comprisespolycrystalline silicon after being heated above said predeterminedtemperature.
 39. The antifuse in claim 34, wherein said end portionscomprise silicide regions of said fin.